The present invention generally relates to an electrical filter, and more particularly, to a dielectric filter in which a plurality of dielectric resonators are coupled by capacitance so as to obtain required band-pass characteristics.
A band-pass filter as referred to above, for example, having a three stage structure, has an equivalent circuit as shown in FIG. 6, and includes a series resonance capacitor C5 connected in series to one of three dielectric resonators 11 connected to ground, and four coupling capacitors C1,C2,C3 and C4 in total, with the junction between each two of said coupling capacitors being respectively connected to one of said dielectric resonators 11 as shown.
FIG. 10 shows a structure of a conventional dielectric filter based on the equivalent circuit of FIG. 6. In the filter of FIG. 10, the coupling capacitors Cl to C4 are formed respectively between five electrodes Ea,Eb,Ec,Ed and Ee formed on the upper surface of a substrate S. Terminal pin P serves as a series resonance capacitor and also is attached (as described below) to an inner conductor Rb of one of the dielectric resonators R. The terminal Pin P is connected at its end Pa, to the central electrode Ec, while terminals T attached to inner conductors Rb of the other dielectric resonators R are respectively connected, at lug portions Ta thereof, to the corresponding neighboring electrodes Eb and Ed. As shown in FIG. 11, each of the terminals T has a base end to be attached to the inner conductor Rb, formed generally into a cylindrical shape, and a forward end thereof formed into a lug portion Ta for connection with the corresponding electrode. Although not particularly shown, terminals for input and output are connected to the electrodes Ea and Ee at the opposite ends of the substrate S for external connections.
As illustrated in FIG. 12, the terminal pin P of a metallic material has its one end Pb inserted into an attaching jig J of a resin material, which is further fitted into the inner conductor Rb of the dielectric resonator R, with its other end Ra projecting outwardly from the jig J of the dielectric material, thereby to form the series resonance capacitor C5 between the end portion Pb and the inner conductor Rb of the dielectric resonator R. The capacitor C5 referred to above functions to sharpen the characteristic at the end of the pass-band region by forming an attenuation pole as shown at A in FIG. 13 by a solid line curve, the normal state being represented by a dotted line, i.e. by polarization as shown in FIG. 13.
However, when the capacitance of the series resonance capacitor C5 is provided by a terminal pin P as described above, there has been the disadvantage that, since the attaching jig J of the resin material is disposed between the metallic pin portion Ra and the inner conductor Rb, the capacitance tends to vary to a large extent depending on temperature, thus resulting in statistical scattering of the frequency characteristics.
In order to overcome the disadvantage as described above, there has also been conventionally proposed an arrangement as shown in FIG. 14, in which another electrode Ef is formed on the substrate S in a position close to the central electrode Ec, which forms part of the coupling capacitors. The series resonance capacitor C5 for polarization is thereby formed between said electrodes Ec' and Ef, and the lug portion Ta of the terminal T attached to the inner conductor Rb of the dielectric resonator R is connected to the electrode Ef. However, in the above case, the problem has been that a large capacitance is not available due to the small area of the electrode Ef, and it would be required to enlarge the substrate in order to obtain a large capacitance by increasing the electrode area, thus undesirably resulting in a large overall size of the filter.